JasonC SBB wrote:If the point on the car that you are grounding via the oscilloscope has a large dv/dt on it, you will introduce "capacitive displacement currents" and the reading will be contaminated.

That's what I thought. Since there is a question of noise on the sensor ground I thought it might find somewhere to hide in a couple of hundred feet of earth wire if I hooked the sensor ground to the scope chassis. Hence hooking a second probe to the sensor ground, with both probes grounded to the car chassis (and house's earth). The effective signal is then the difference in the probe voltages.

Yes but the scope's sampling rate will affect your measurement. If the subtraction is done in continuous time using an analog circuit, you will get the right answer. If the subtraction is done digitally after the signals are digitized, (as is the case with your DSO), any narrow spikes that appear in both signals, but which happens to be "seen" by one channel but missed by the other (because said spike is narrower than one sampling period), will falsely appear in the subtracted signals.

In practice the dv/dt on signal ground and on chassis ground (relative to the "universe") are benign enough that you can just clip the probe to either ground (depending on what you are measuring), and get a good measurement. What you cannot do is place one ground clip on one point and another ground clip on another point, if you want an accurate measurement of one signal. If the 2 points are at different potentials that will drive current up the scope probe shields, distorting your measurements.

while you and piledriver emphasise that "ground" is an artificial concept, it does still have to be a fixed voltage doesn't it?

Relatively, yes.

You couldn't hook the ground clip to a carrier waveform and read the carried signal at the probe could you? Even if nothing melted, the waveform would be corrupted. Right?

Correct, with one possible exception. The whole oscilloscope is floated by powering via a high-isolation transformer, which places its chassis at a high impedance vs. the "universe". It's a standard trick I do with lightning surge testing.

piledriver wrote:Note there are several diferent standard ground symbols for a reason...It's not at all unusual to see a schematic with at least two or three different groundplanes, each with a different symbol.

True, but I can't tell you how many times customer engineers then mis-interpret the schematic and mix em all up, DESPITE all the admonitions and descriptions in the Application Notes I spent months of my life writing!

Peter Florance wrote:Further I have to repeat that as long as sensor ground wire does not carry current, there should not be a issue with noise or voltage offset on it. But if you connect the sensor ground wire to vehicle, you are asking it to help sink current from the MS box and it is no longer a reference, but a conductor.

Correct. But said currents would be the sensor currents (e.g. MAP sensor current as pointed out), and not the large injector/solenoid currents. What makes said large currents even more corrupting is that they have large di/dt.

piledriver wrote:Ideally, If your mainboards ground planes were split up properly, and/or if you have all external power drivers for coils/injectors, the MAP/TPS/etc sensors go to their own dedicated signal ground pin(s).

Analog circuitry and the A/D have to be on the same ground plane.The high current stuff can be on another.

Any signals that go from one to the other have to be routed close to the point where the 2 ground planes are connected, whether said connection is a polyfuse or a hard connection.See the "Ott" webpage I linked.Additionally said signals that cross need to have receivers that are noise immune, such as by using Schmitt triggers for digital signals, and differential receivers for analog signals.

1031 wrote:Another point that has pointed out was adding capacitance after polyfuse, yes i tested that, whit normal electrolyts (good quality Panasonic FM series and also Sanyo´s OS-CON´s that have wery low ESR at wide freq.range) but none of those worked as replacing that polyfuse by normal fuse. So i must say that at this case noise was related only to Vref´s impedance (series resistance of that polyfuse)

Where is the 5V regulator ground pin, and where did you attach the - side of the Panny and Oscon caps? How long were the leads?

JasonC SBB wrote:It's just that I was told elsewhere that the polyfuse sat between signal gnd and power gnd.In any case, we've been discussing principles and not just specifics. That's not "speculation".

JasonC SBB wrote:It's just that I was told elsewhere that the polyfuse sat between signal gnd and power gnd.In any case, we've been discussing principles and not just specifics. That's not "speculation".

Given the comments about caps after the polyfuse, I find it interesting that Bruce Bowling has suggested the following add-on circuit for noisy VRef:

Reference Voltage (Vref) Protection Circuit

There are some external sensors which require 5 volts to operate, namely the MAP, MAF, and TPS sensors. This reference voltage is generated in the Module and supplied externally thru the Vref signal and Sensor Ground. In order to maintain ratiometric processor ADC operation the same supply is used for both ADC reference and sensor power. This means that there is a possibility of noise coupling back in from the Vref signal to the Module.

The circuit below shows a possible filtering arrangement for Vref. First off, adding a large capacitance on Vref will help make it steady and also keep the Vref line stiff at 5V. Anything from 47 microFarads to hundreds of µF can be used. Next, a simple inductor/capacitor arrangement can be of help. Since there is very little current draw on the sensor ground and the Vref supply is stiff, adding inductance here is not an issue. You can also substitute a pre-made EMI filter like the Murata BNX002-01 filter here if desired.

JasonC SBB wrote:It's just that I was told elsewhere that the polyfuse sat between signal gnd and power gnd.In any case, we've been discussing principles and not just specifics. That's not "speculation".

That is why you should look at the schematics yourself.

I did and I couldn't find the polyfuses at the time, so I relied on what I had heard.

JasonC SBB wrote:It's just that I was told elsewhere that the polyfuse sat between signal gnd and power gnd.In any case, we've been discussing principles and not just specifics. That's not "speculation".

I'm concerned that we are causing users to expend a lot of energy on possibly redesigning the MS board when all they have to do is install like this:

robs wrote:That's what I thought. Since there is a question of noise on the sensor ground I thought it might find somewhere to hide in a couple of hundred feet of earth wire if I hooked the sensor ground to the scope chassis. Hence hooking a second probe to the sensor ground, with both probes grounded to the car chassis (and house's earth). The effective signal is then the difference in the probe voltages.

Yes but the scope's sampling rate will affect your measurement. If the subtraction is done in continuous time using an analog circuit, you will get the right answer. If the subtraction is done digitally after the signals are digitized, (as is the case with your DSO), any narrow spikes that appear in both signals, but which happens to be "seen" by one channel but missed by the other (because said spike is narrower than one sampling period), will falsely appear in the subtracted signals.

I was planning on doing the subtracting myself on the capture, but I can see the same problem applies. This is down to my ignorance of oscilloscopes. I thought both channels would do something like a capture and hold at the same tick of the clock. Perhaps it's asking a bit much to do that at 1G samples/s.

JasonC SBB wrote:

You couldn't hook the ground clip to a carrier waveform and read the carried signal at the probe could you? Even if nothing melted, the waveform would be corrupted. Right?

Correct, with one possible exception. The whole oscilloscope is floated by powering via a high-isolation transformer, which places its chassis at a high impedance vs. the "universe". It's a standard trick I do with lightning surge testing.

Think I'll leave voltages like that to people like Dr Frankenstein and your good self.

1031 wrote:Another point that has pointed out was adding capacitance after polyfuse, yes i tested that, whit normal electrolyts (good quality Panasonic FM series and also Sanyo´s OS-CON´s that have wery low ESR at wide freq.range) but none of those worked as replacing that polyfuse by normal fuse. So i must say that at this case noise was related only to Vref´s impedance (series resistance of that polyfuse)

Where is the 5V regulator ground pin, and where did you attach the - side of the Panny and Oscon caps? How long were the leads?

I´m aware of Inductance of long cap leads etc.. so i used shortest connection possible.. Point was here to tell that capacitance after polyfuse (cap between polyfuses Vref side and sensor GND return) did not helped whit that noise. Jason you shoud read these post´s more carefully and look at those scematics/pcb layout pictures.. many of thise thing´s have posted before.

Last edited by 1031 on Fri Apr 13, 2012 4:29 am, edited 1 time in total.

I want to also state, if your factory harness does not match above scheme, you may have problems when you install a MegaSquirt using that harness.If you unplug your MegaSquirt box and still have continuity to ground from your sensor commons, you might have an grounding scheme issue.

Peter - when posting something as critical as that grounding diagram, please include a link to the manual page - it's a new page, and I for one hadn't seen it before (my wife says that means absolutely nothing - thank you for the support, dear ). There's a little more info there that makes it a lot clearer. Thank you for bringing it up, though!